1. Field of the Invention
The present invention relates to a glass substrate for information recording media and a manufacturing method thereof, and in particular to a glass substrate for information recording media used in information recording devices such as a hard disk drive and a manufacturing method thereof.
2. Prior Art
In recent years, there has been remarkable progress in digitalization of information, and various types of information recording device for recording such digital information have been developed and manufactured. These devices are being improved at a rapid rate, with the information storage capacity and the recording and playback speeds increasing at a rate of over 10% per year. The most widely used information recording devices are currently hard disk drives, and the rate of improvement of hard disk drives is faster than that of other devices.
In a hard disk drive, recording and playback of information is carried out to and from an information recording layer formed on an information recording medium substrate (hereinafter merely referred to as the xe2x80x9csubstratexe2x80x9d) using a magnetic head. Types of hard disk drive that are widely used at present are xe2x80x9cCSS (contact start/stop) typexe2x80x9d and xe2x80x9cramp load typexe2x80x9d. In the CSS type, the magnetic head flies over a data zone of the substrate while the substrate is rotating, and slides over a CSS zone of the substrate when the substrate starts or stops moving. The CSS zone of the substrate is a portion of the substrate (generally provided along the inside perimeter or the outside perimeter of the substrate) on which uniform undulations of height several tens of nm are intentionally provided. In the ramp load type, the magnetic head flies over the substrate while the substrate is rotating, and is stored in a storage position when the substrate stops. With the ramp load type, because of the setup involved, a CSS zone is not needed. Moreover, in recent years, studies have been carried out into a contact type hard disk drive in which the magnetic head and the substrate are always in contact with one another.
In CSS type and ramp load type hard disk drives, while the substrate is rotating, the magnetic head flies over the substrate with a gap (hereinafter referred to as the xe2x80x98flying heightxe2x80x99) between the magnetic head and the substrate surface (information recording region) of several tens of nm. To realize a high recording density, it is necessary to reduce the flying height. However, if the surface undulations of the substrate are large, then the magnetic head will collide with projections on the substrate surface during rotation, increasing the possibility of a head crash occurring. Moreover, even before a head crash occurs, there is a risk of occurrence of so-called thermal asperity, in which heat caused by such collisions results in the magnetic head detecting an abnormal signal and hence malfunctioning. In particular, in recent times high-sensitivity MR heads and GMR heads have become the mainstream, and as a result the problem of thermal asperity has become yet more serious.
Conventionally, it was thought that head crashes and thermal asperity caused by collisions between the magnetic head and projections on the substrate surface could be avoided by reducing the undulations on the substrate surface in the information recording region, and thus that the higher the surface smoothness, the better the performance of the substrate. Representative examples of methods that have been proposed for removing surface undulations and forming a smooth surface include a method in which a smooth substrate is formed by carrying out finishing polishing using artificial suede, i.e. by removing a surface layer (Japanese Laid-open Patent Publication (Kokai) No. 2000-53450), and a method in which surface scrubbing is carried out using a sponge made of PVA or the like, thus removing foreign matter and surface undulations and hence smoothing the substrate (Japanese Laid-open Patent Publication (Kokai) No. 2000-149249).
However, there are problems with the conventional art such as the following.
Through detailed studies, the present inventors have discovered that the frequency of occurrence of head crashes can indeed be reduced by increasing the surface smoothness of the substrate, but that if the surface undulations are made too small, then the flight stability of the magnetic head drops, resulting in the frequency of collisions between the magnetic head and the substrate surface increasing, and hence in the risk of occurrence of a head crash increasing. The cause of this drop in the flight stability of the magnetic head is not clear, but it is thought that it is because the magnetic head wobbles as it flies over the information recording region, resulting in contact with the substrate being unavoidable, and hence in the frictional force rising unless there are moderate surface undulations (hereinafter referred to as xe2x80x9ctexturexe2x80x9d). In particular, nowadays the flying height is set very low to increase the recording density, and hence it is thought that head crashes caused by wobbling of the magnetic head are yet more prone to occur. It is thus necessary to ensure that the magnetic head is not subjected to an excessive resistance when the flying magnetic head contacts the substrate. Moderately roughening (hereinafter referred to as xe2x80x9ctexturingxe2x80x9d) the substrate surface can be envisaged as a means for achieving this.
However, with a substrate that has been subjected to texturing, there is a problem in that it is difficult to selectively remove abnormal projections or foreign matter attached to the substrate surface. For example, if surface scrubbing is carried out using a soft sponge such as a polyvinyl formal sponge or a polyurethane sponge, then it is difficult to remove abnormal projections that have been formed during the texturing or foreign matter attached firmly to the substrate surface, and if the abnormal projections are forcibly removed under harsh scrubbing conditions of a long time, a high pressure and a high speed of rotation, then the texture itself may be excessively worn down. Moreover, in the case of polishing using a polishing machine or finishing polishing, there is a problem in that the substrate surface is removed, and hence the texture may again be excessively worn down.
In view of the problems described above, it is an object of the present invention to provide a glass substrate for information recording media in which the undulations on the glass substrate surface are optimized, thus contributing to reduction of the flying height, and at the same time preventing the occurrence of head crashes and thermal asperity, and a method of manufacturing the glass substrate.
To attain the above object, the present invention provides a method of manufacturing a glass substrate for an information recording medium, comprising the steps of polishing at least one surface of a glass substrate, and subjecting the polished at least one surface of the glass substrate to surface scrubbing using a sponge having an Asker C hardness of not less than 40 according to The Society of Rubber Industry, Japan SRIS 0101.
According to the method of the present invention, at least one polished surface of a glass substrate is subjected to surface scrubbing using a sponge having an Asker C hardness of not less than 40 according to The Society of Rubber Industry, Japan Standard SRIS 0101. As a result, the advantages of a sponge resulting from the part where the sponge comes into contact with the glass substrate being hard on a microscopic scale but pliant on a macroscopic scale, namely the advantages that the glass substrate is not easily scratched even if foreign matter is present, and any slight loss of parallelity between the sponge and the glass substrate is mitigated by the pliancy of the sponge are retained, and abnormal projections caused by ununiform etching of the glass substrate and firmly attached foreign matter can be removed, and hence a texture free of abnormal projections and having a uniform projection height can be produced. As a result, when the glass substrate is used as the substrate of an information recording medium, the flying height can be reduced, and the occurrence of head crashes and thermal asperity can be prevented.
Preferably, the sponge comprises an underlayer and a surface layer, with the Asker C hardness of the surface layer being not less than 40. As a result, the above-mentioned effects according to the present invention can be exhibited reliably.
More preferably, the surface layer of the sponge comprises a resin having a 100% modulus of not less than 45 kg. As a result, abnormal projections can be removed without excessively wearing down the texture on the glass substrate surface.
Still more preferably, the resin is a polycarbonate type polyurethane resin. As a result, the chemical resistance of the sponge can be improved.
Preferably, the sponge has a surface layer comprising a spongy body having a mean opening diameter of not less than 30 xcexcm. As a result, abnormal projections can be removed well without excessively wearing down the texture on the glass substrate surface.
Preferably, the surface scrubbing is carried out using an alkaline aqueous solution of not less than 8 pH. As a result, electrostatic repulsive forces act between the removed abnormal projections and the glass substrate, and hence reattachment of the removed abnormal projections can be prevented.
Preferably, the surface scrubbing is carried out using an acidic aqueous solution of not more than 5 pH. As a result, metallic impurities can be dissolved effectively and hence removed.
Preferably, the surface scrubbing is carried out on the at least one surface of the glass substrate after the at least one surface of the glass substrate has been subjected to texturing. As a result, abnormal projections can be removed more reliably.
More preferably, the at least one surface of the glass substrate that has been subjected to the surface scrubbing is then subjected to chemical strengthening. As a result, a high-strength glass substrate having a compressive stress layer formed on the at least one surface thereof can be produced.
Also preferably, the surface scrubbing is carried out on the at least one surface of the glass substrate after the at least one surface of the glass substrate has been subjected to texturing and chemical strengthening in this order. As a result, the texture formed on the at least one substrate surface can be prevented from being excessively worn down during polishing using a polishing machine or finishing polishing.
To attain the above object, the present invention also provides a glass substrate for an information recording medium manufactured using the method according to the present invention as described above, wherein a bearing height BH04 of at least one surface of the glass substrate at which a contact ratio is 0.4% as measured using an atomic force microscope is in a range of 2 to 7 xcexcm.
According to this glass substrate for information recording media, the bearing height BH04 of at least one surface of the glass substrate at which the contact ratio is 0.4% as measured using an atomic force microscope is in a range of 2 to 7 xcexcm. As a result, a texture free of abnormal projections and having a uniform projection height can be produced, and hence when the glass substrate is used as the substrate of an information recording medium, the flying height can be reduced, and the occurrence of head crashes and thermal asperity can be prevented.
To attain the above object, the present invention also provides a glass substrate for an information recording medium manufactured using the method according to the present invention as described above, wherein a bearing height BH01 of at least one surface of the glass substrate at which a contact ratio is 0.1% as measured using an atomic force microscope is in a range of 2 to 10 xcexcm.
According to this glass substrate for information recording media, the bearing height BH01 of at least one of the glass substrate at which the contact ratio is 0.1% as measured using an atomic force microscope is in a range of 2 to 10 xcexcm. As a result, a texture free of abnormal projections and having a uniform projection height can be produced, and hence when the glass substrate is used as the substrate of an information recording medium, the flying height can be reduced, and the occurrence of head crashes and thermal asperity can be prevented.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.